Moonlit mysteries

The recent discovery of polar ice sheets on the moon reminds us that satellite Selene still has her secrets, but another of her mysteries has been visible from Earth for almost 500 years.

In 1540, more than half a century before the first telescope was unveiled, stargazers in the German city of Worms saw a star-like object appear in the moon's north-eastern Calippus region. The Polish astronomer Hevelius in 1650 noted the appearance of a Mons Porphyrites, or "red hill", in the north-western crater Aristarchus. In April 1787, the astronomer Sir William Herschel saw so many red lights in this same crater that he thought he was witnessing a lunar volcano, and invited George III to take a look.

More recently, Nasa records reveal several hundred spots and flashes of light, coloured glows, mists, unusual shapes and shadows have been observed on the lunar surface. They are known collectively as Transient Lunar Phenomena (TLP), a term coined by Sir Patrick Moore.

Professional astronomers sniffed at these reports for many years, declaring them optical illusions or telescope errors. But a 1968 Nasa catalogue documented hundreds of sightings, the vast majority in and around Aristarchus. In July the following year, as Apollo 11 entered lunar orbit, earthbound German astronomers reported seeing a bright TLP in Aristarchus. Asked to check it out, Neil Armstrong noted unusual illumination in what he took to be the same crater.

So is the moon, supposedly a lifeless chunk of basaltic rock, undergoing periodic volcanic activity? Most astronomers don't think so. In1963 Zdenek Kopal observed that a significant TLP event occurred during a major solar flare-up. He suggested particles from these vast electromagnetic emissions caused moon rocks to glow, particularly when the moon is passing through the Earth's magnetosphere.

Another theory was put forward by Audouin Dollfus of the Paris Observatory in 1992. He identified glowing clouds emerging from Langrenus as moon dust, thrown up by underground gaseous emissions. The crater's surface is indeed cracked, and radon gas was detected in the atmosphere during the Apollo missions. Perhaps these moon burps are the source of some TLP, but it'll take another lunar landing to be sure.